27th European Photovoltaic Solar Energy Conference and Exhibition
VARIATION OF THE POWER AS A FUNCTION OF MODULE TEMPERATURE FOR DIFFERENT THIN FILM MODULES C. Cañete, R. Moreno, J. Carretero, M. Piliougine, M. Sidrach-de-Cardona Departamento de Física Aplicada II, Universidad de Málaga, Louis Pasteur 35, 29071 Málaga, Spain. Tel: +34952132772. Email:
[email protected] ABSTRACT: The aim of this study is to determine the evolution of the power of photovoltaic modules of different technologies and their temperature dependence through one year. For this purpose, photovoltaic modules of CdTe, a-Si, a-Si/c-Si and pc-Si have been installed under outdoor conditions on the roof of the photovoltaic laboratory at the University of Málaga, Spain. The obtained results show a variability of the peak power that is a function of the variation of the incident irradiance spectrum. However, this influence on the measured peak power is different for each technology. Thin film modules have higher peak power variability throughout the year than conventional modules. The amorphous silicon module presents the greatest annual variability, reaching 7.4% its nominal power. The polycrystalline silicon module is the most stable with an annual variation of 1.5% of its nominal power. No irreversible damage has appreciated in the tested modules during this period. We have determined the power thermal coefficient for clear-sky days throughout the year. The results show that the variation of this coefficient depends on the temperature of the module. Keywords: PV Module, Reliability, Thin Film 1
There are many authors who have studied the behaviour of polycrystalline PV modules under real conditions in order to understand the meteorological influence on output performance. Huld et al. [5] propose a model to predict the polycrystalline PV module output performance depending on module temperature and irradiance. So they make a comparison between outdoor and indoor measurement to conclude that polycrystalline modules suffer degradation on performance when they have been exposed for a certain period of time under outdoor conditions. The annual average degradation for polycrystalline modules was 0.7%. An approximate value of performance degradation was found for 204 polycrystalline modules exposed during 21 years in Ispra (Italy) by Shocked et al. [6]. Amorphous silicon (a-Si) exhibits a light-induced degradation phenomenon known as Staebler-Wronskly Effect (SWE). It is observed in the first period of outdoor exposure. Ishii et al. [7] studied the behaviour of different kinds of silicon modules. As a result, a reduction on the a-Si output performance was found in the first year of exposure due to SWE. A positive value of maximum power temperature coefficient () has also been found. Despite of the value given by manufacturers for this coefficient is always negative; a positive value can be due to a combination of solar spectral distribution and annealing effect. On the other hand, CdTe modules show different behaviour. Under the same conditions of exposure, some CdTe modules show an increase of power output in the first period of exposure (6-8%) and another ones show degradation (7-15%) in the same period. It depends on the specific manufacture process [8].
INTRODUCTION
The market penetration of new photovoltaic thin film technologies has stressed the importance of knowing how they perform under outdoor conditions. According to the European Photovoltaic Industry Association (EPIA), the global production capacity of thin film modules will be between 6 and 8.5 GW in 2012 [1]. The output power of a PV module is generally calculated at standard test conditions (STC), which correspond to 1 kW/m2 of solar irradiance, 25 °C of module temperature and a spectrum AM1.5. The experimental determination of the peak power of a module at STC can be made by measuring the I-V curve both with a solar simulator and under outdoor conditions. Obviously, in this latter case the correction of the experimental curve is required. The experimental measurement under outdoor conditions is affected not only by the irradiance and temperature values, but also by the angle of incidence and the spectral distribution of the incident irradiance. The effect of the spectral distribution is different for each module technology, as it has been described by other authors [2]. Furthermore, according to previous results, photovoltaic modules under outdoor exposure conditions can suffer degradation processes, both short term and long term, decreasing their initial peak power. The aim of this study is to determine the evolution of the output power of different PV module technologies and its dependence on temperature during one year of exposure under outdoor conditions. For this purpose, several photovoltaic modules of CdTe, a-Si, a-Si/c-Si and pc-Si have been installed on the roof of the photovoltaic laboratory at the University of Málaga, Spain.
3. EXPERIMENTAL SETUP 2. PREVIOUS WORKS 3.1 Measurement system The performance of the modules was calculated by measuring the I-V characteristic curves under outdoor conditions at 3 minutes intervals. The measurement system is based on a four-quadrant power supply, which is connected to the modules in a four-wire configuration.
Previous works analyse the behaviour of different photovoltaic thin film technologies under outdoor conditions and the influence of irradiance, module temperature, spectral distribution and angle of incidence on the module output performance [3,4].
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27th European Photovoltaic Solar Energy Conference and Exhibition
Module voltage and current are measured with a pair of digital multimeters connected to a PC. At the same time that the I-V curve is measured, meteorological parameters are acquired, such as irradiance on module plane, module temperature and air temperature, among others. All sensors are connected to an acquisition data system connected to the control computer. A complete description of the experimental system is given in a previous work [9]. The module temperature was measured with a RTD Pt100 sensor coupled to its backside with thermal paste and the air temperature was measured with a Pt100 sensor from a weather station close to the modules. Irradiance level on the module plane was measured using a pyranometer Kipp & Zonen CMP21. A spectroradiometer EKO MS-710 (spectral response from 350 nm to 1050 nm) is used for measuring the spectral distribution of the solar incident irradiance.
according to the expression: G Pi (1000 W/m2 ) STC Gi
4. RESULTS AND DISCUSSION For each month, the values of peak power versus irradiance at standard temperature conditions for a pc-Si photovoltaic module are shown in Fig. 1. As it can be observed, the peak power at STC for this module is kept stable along the period of study (one year). The average value of the power thermal coefficient obtained during the winter months (–0.78 W/°C) is close to the value provided by the manufacturer (–0.741 W/°C) . However, during the summer months higher experimental values of this coefficient are obtained (–1.09 W/°C) . These differences may be due to the fact that during summer months the modules operate at very high temperatures. This implies that the values of peak power calculated according to eq. (2) are somewhat higher than those obtained from eq. (1). Based on these results we can say that the peak power of the p-Si module is fairly stable throughout the year and therefore, the influence of the solar spectrum is small. The observed variation of peak power depending on time of the year is about 3 W. This value represents 1.5% of its nominal power. In the PV modules with thin film technologies the variation of the peak power along the year has been greater. In Fig. 2 and in Fig. 3 theses differences can be observed for a-Si/µc-Si and a-Si modules. In these cases the values obtained for the power thermal coefficient have been different each month. These values are always higher in summer than in winter.
Table I: Performance of installed PV modules CdTe 9.7 0.72 70.0
a-Si a-Si/µc-Si 6.3 8.5 0.95 1.49 60.0 121.0
pc-Si 13.4 1.46 195.0
The modules were installed in a fixed structure on the roof of the photovoltaic laboratory at the University of Málaga (latitude: 36°43’ N, longitude: 4°25’ W). They were orientated towards South with a tilt angle of 20°. Before starting experimental measures, the modules have been exposed to outdoor conditions for two months. In this way, initial degradation process of the modules has not been considered in this work. 3.3 Determination of the peak power The peak power of the modules has been determined from the experimental data by selecting a clear-sky day for each month. For each day we have only selected values recorded around solar noon, so that the influence of the angle of incidence is minimal. The peak power at 25 °C is calculated according to the following expression
Pi Pi ( 25C) 1 γ Tm ( 25C) Tm
(2)
where Gi is the incident irradiance on the module. If the power is plotted as a function of the module temperature, the slope of this linear fitting represents the experimental value of power thermal coefficient γ. With this value it is possible to recalculate the peak power of the module. The two described methods should provide similar results.
3.2 PV modules used in this study Different types of photovoltaic module technologies have been studied in this paper. In particular, four different photovoltaic modules technologies have been measured: polycrystalline silicon (pc-Si), amorphous silicon (a-Si), tandem of a-Si/µc-Si and cadmium telluride (CdTe). The summary of the electrical characteristics of the each module is shown in Table I.
Technology Efficiency (%) Area (m2) Peak Power (W)
Pi
(1)
where Pi is the measured power, Tm is the module temperature and (W/°C) is the power thermal coefficient which is given by manufacturer. A linear fitting of the power at 25 °C versus solar irradiance allow us to determine the peak power of the module in standard conditions (STC) and its monthly variation. Alternatively it is possible to determine the peak power of the module calculating in advance the thermal coefficient of power. For this purpose the power values at standard irradiance condition have been calculated
Figure 1: Evolution of the peak power translated to standard temperature condition versus irradiance for a pc-Si photovoltaic module.
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27th European Photovoltaic Solar Energy Conference and Exhibition
Figure 2: Evolution of the peak power translated to standard temperature condition versus irradiance for a a-Si/µc-Si photovoltaic module.
Figure 3: Evolution of the peak power translated to standard temperature condition versus irradiance for a a-Si photovoltaic module.
Figure 4: Evolution of the peak power values at STC for different modules technologies throughout the year calculated according with eq. (1) or with eq. (2).
Figure 5: Values of the average photon energy (APE) for different clear-sky days at solar noon in Málaga.
The values obtained for the peak power using the two proposed methods have been similar, as it is shown in Fig. 2. The annual variability of the peak power of the a-Si/µc-Si module was 5 W whereas for a-Si module was 8 W, which represents 4.1% and 8.3% of their nominal peak power values respectively. These results are shown in Fig. 4. The CdTe module shows a similar behaviour that the previous modules with an annual variability of the peak power of 5 W (7.1%) Table 2 and Table 3 summarises the most significant results obtained in this work. Table 2 shows the annual mean values of peak power at STC for each module and its variability throughout the year, both in Wp and in percentage of its nominal power. Note that thin film modules have higher variability throughout the year. This is due to the greater influence of the variation of the solar spectrum on these technologies.
Fig. 5 shows the values of the average photon energy APE (eV) at solar noon for the days used in the determination of the peak power (all of them are the clear-sky days). It can be observed that for winter months the value of APE was around 1.86 eV whereas for summer months this value was around 1.91 eV. Table II: Experimental values of the peak power and its yearly variability Peak power Peak power STC (Wp) variability Manufacturer Measured Wp % 195 192 pc-Si 3 1.5 121 112 5 4.1 a-Si/c-Si 60 57 a-Si 5 8.3 70 67 CdTe 5 7.1
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27th European Photovoltaic Solar Energy Conference and Exhibition
Table III: Experimental values of the power thermal coefficient
REFERENCES [1] Global Market Outlook for Photovoltaic until 2015. European Photovoltaic Industry Association (EPIA), May 2011. [2] R. Gottschalg, T.R. Betts, D.G. Infield, M.J. Kearney, Measurement Science and Technology 15 (2004) 460. [3] J. Kurnik, M. Jankovec, D. Brecl, M. Topic, Solar Energy Materials & Solar Cells 95 (2011) 373. [4] M. Topic, K. Brecl, J. Sites, Progress in Photovoltaic: Research and Applications 15 (2007) 19. [5] T. Huld, G. Friesen, A. Skoczek, R.P. Kenny, T. Sample, M. Field, E.D. Dunlop, Solar Energy Materials and Solar Cells 95 (2011) 3359. [6] A. Skocked, T. Sample, E.D. Dunlop. Progress in Photovoltaics: Research and Applications 17 (2009) 227. [7] T. Ishii, T. Takashima, D. Otani, Progress in Photovoltaic: Research and Applications 19 (2011) 170. [8] M. Gostein, L. Dunn, Proceedings of the 37th IEEE Photovoltaic Specialists Conference (2011) 3126. [9] M. Piliougine, J. Carretero, L. Mora-López, M. Sidrach-de-Cardona, Progress in Photovoltaic: Research and Applications 19 (2011) 591.
Power thermal coefficient (W/°C) Manufacturer Average Winter Summer –0.74 –0.91 –0.79 –1.09 pc-Si –0.29 –0.23 –0.17 –0.30 a-Si/c-Si –0.138 –0.12 –0.09 –0.16 a-Si –0.175 –0.24 –0.18 –0.33 CdTe This explains why during the winter months the value obtained from the peak power of the modules is lower and why this value is higher for summer months. The values obtained for the power thermal coefficient are compared in Table III. It is possible to observe that for all modules the value of γ is not constant being greater in summer than in winter. 5 CONCLUSIONS In this work we have studied the behaviour of the peak power for modules of different technologies for a year. The obtained results show that the peak power of the modules under outdoor conditions is strongly dependent on the season of the year. This is due to the variations of the incident irradiance spectrum throughout the year. In measurements made in winter the values obtained for the peak power are lower than in those made in summer. In our latitude, the incident spectrum is more energetic in summer than in winter, as it can be seen from measured values of APE. The variability for thin film modules is greater than for pc-Si modules. Thus a-Si and CdTe have the greatest variations throughout the year. For example, for the a-Si module the values estimated for the peak power can differ up to 8.3% of its rated power. This value is only up to 1.5% in the pc-Si module. For the a-Si/µc-Si this variability is up to 4.1% and 7.1% for the CdTe. This means that the efficiency of thin film modules is very dependent on the spectrum of the incident irradiance. This variability is seasonal. The experimental values of the power thermal coefficient show that this ratio cannot be considered constant throughout the year, for any of the technologies studied. In all cases the value of this coefficient is lower in the winter months (cold) than in summer months (warm) suggesting that this coefficient depends on the module temperature. We note that the values obtained are in some cases far from the value supplied by the manufacturer. Finally, it is noteworthy that irreversible damage in the modules has not been appreciated for the period of study. ACKNOWLEDGEMENTS We acknowledge to the “Junta de Andalucía” (grant No. P07-RNM-02504) for financial support.
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